Japanese binoculars of the 70-80's with A/K prisms? (1 Viewer)

[pat mitchel]

I am aware of one specific Japanese binocular with Abbe-Koenig prisms that was made for Tasco in the #490R model 8x56 (6deg) in 1976. Is anyone aware of others? Further, does anyone have "eyeball time" with the Tasco? Given that early AK's don't have the dielectric coatings that the SP prisms have, I was wondering how bright or not the image was. Thanks, Pat

 

[Mark9473]

When a decade ago even the original Zeiss Conquest series, which had AK prisms, weren't particularly bright or contrasty, I have little hope for even older models.

 

[pat mitchel]

Well, crap. Another pipedream down on flames (weird metaphor, eh?) Thanks Mark9473. Regards, Pat (who still keeps looking!)

 

[jring]

Hi,

no Abbe-Koenig prism has dielectric coatings as they don't need them because all internal reflections are at an angle that allows for total internal reflection.
That's one of the crutches used to fix many problems of Schmidt-Pechan prisms...

They do have a roof edge though, and that needs phase coatings or loose some contrast due to destructive interference of the two half beams...

Joachim

 

[jan van daalen]

OK, for those who don't understand the reason for the destructive interference I'll try to explain:

See the roof prism as a house drawn by a child. Two vertical stripes (the walls) and two 45 degrees stripes (the roof). The lightbeam comes from the basement and hits the wall you look at. So not the wall drawn by the vertical stripes but the "virtual" wall you look at on the back of that house on which it gets reflected to the roof. Where the 45 degrees angle starts the beam goes 90 degrees to the left and on the other side of the roof to the right and goes reflected down again. Higher up on the roof the optical path is much smaller because the distance in the top between the two roofsides is shorter, causing the wavelenght to change half a phase. This is called the destructive interference of the two half beams. By applying a phasecoating on one side of the roof edge the phase shift is eliminated causing higher "contrast".

This was first discovered by Zeiss and applied on their bins in 1987 and marked as *P on those bins. So first came *T in 1978 (T for Transmissionsteicherung).

Jan